This invention relates to magnetic recording disk drives for use with patterned media, wherein each data bit is stored in a magnetically isolated block on the disk.
Conventional magnetic recording disks are made with a continuous magnetic layer that is deposited on an aluminum alloy or glass substrate coated with a nickel-phosphorus layer (referred to hereafter as continuous magnetic disk media). The magnetic materials, or media, are generally cobalt, nickel and/or iron alloys deposited by evaporation or sputtering to form the continuous magnetic layer. In such media, each magnetic bit consists of several hundred small grains of the magnetic alloy material. One approach to increasing the bit density for conventional continuous magnetic disk media is to reduce the grain sizes along with the bit sizes while keeping the total number of grains per bit approximately constant. This approach is limited because very small grains may switch magnetization direction spontaneously at normal operating temperatures because their magnetic energy is comparable with that available thermally. The alternative approach of reducing the number of grains per bit leads to poor recording performance due to an increase in the noise arising from statistical fluctuations in grain positions or orientation.
The use of patterned magnetic disk media is one approach to overcome the problems associated with increasing bit densities by reducing grains size. Increasing bit densities of magnetic recording media can be achieved by patterning the magnetic material into small isolated islands or blocks such that there is a single magnetic domain in each block or xe2x80x9cbitxe2x80x9d. The single magnetic domains can be a single grain or consist of a few strongly coupled grains that switch magnetic states in concert as a single magnetic volume. With only a single magnetic volume per block, noise fluctuations arising from grain positions or orientation are eliminated. To produce the required magnetic isolation of the patterned blocks, the magnetic moment of the regions between the blocks must be destroyed or substantially reduced so as to render these regions essentially nonmagnetic. Alternatively, the media may be fabricated so that that there is no magnetic material in the regions between the blocks. U.S. Pat. No. 5,820,769 is representative of various types of patterned media and their methods of fabrication. A description of magnetic recording systems with patterned media and their associated challenges is presented by R. L. White et al., xe2x80x9cPatterned Media: A Viable Route to 50 Gbit/in2 and Up for Magnetic Recording?xe2x80x9d, IEEE Transactions on Magnetics, Vol. 33, No. 1, January 1997, 990-995.
In conventional magnetic recording where the data bits are written on continuous media, there is no requirement to write to precise positions on the media since all of the media contains magnetic material. However, to write on patterned media using the conventional unsynchronized approach, the media must be patterned perfectly with a single accurate period, and the effective motor speed of the spindle supporting the disks must be highly stable. Together, the accuracy of the media patterning and the stability of the spindle speed has to be such that bits could be written over distances up to 1 mm with positioning accuracy of the bits to about 10 nm. In the White et al. article it is suggested that the conventional read/write head could not be modified to allow reading of synchronization or clocking marks by the read head while writing occurs by the write head because of the significant coupling between the read and write signals.
Prior to the interest in patterned media, patterned xe2x80x9cdiscrete trackxe2x80x9d media was proposed, as described in IBM""s U.S. Pat. No. 4,912,585. In this type of media, each data track consists of continuous media, like the conventional media, but the individual data tracks are separated by nonmagnetic guard bands. In addition, special marks, such as servo positioning marks and synchronization marks indicating the beginning of a data block, are formed as discrete magnetic blocks separated by nonmagnetic regions. The reading of synchronization or clocking marks by the read head of a conventional read/write (R/W) head in a magnetic recording system that used discrete tracks of continuous media separated by nonmagnetic guard bands has been demonstrated by H. Yada, et al., xe2x80x9cExternal Clocking PRML Magnetic Recording Channel for Discrete Track Mediaxe2x80x9d, IEEE Trans. Fundamentals, Vol. E76-A, No. 7, July 1993, 1164-1166. In that system, the clocking marks were read from discrete magnetized regions in servo/clocking sectors spaced along the tracks, with the user data being written in the continuous magnetic media located between the servo/clocking sectors.
What is needed is magnetic recording system for patterned media that compensates for imperfect patterning of the media by modifying the timing of the write pulses.
The invention is a magnetic recording disk drive that uses patterned disk media wherein discrete magnetic data blocks representative of the individual data bits are isolated from one another. The carrier for the read/write head includes a special pattern sensor that senses the data blocks in the data tracks before they pass beneath the write head. The pattern sensor output serves as the clocking signal to precisely control the placement of the write pulses by the write head. A time delay is calculated using a timing mark on the patterned disk to delay the write pulses so that a data block sensed by the pattern sensor is the same data block to which the write pulse is applied. In this manner the actual previously recorded data provides the synchronization or clocking signal to control the writing of the new data. The time delay is calculated from measurement of the time for a timing mark to pass from the pattern sensor to the read head and from known spacings of the pattern sensor, read head and write head on the head carrier. The pattern sensor may be a magnetoresistive-type sensor, a capacitive sensor that senses capacitive contrast between the data blocks and the nonmagnetic regions, or a thermal sensor that senses variations in thermal conductivity between the data blocks and the nonmagnetic regions.
For a fuller understanding of the nature and advantages of the present invention, reference should be made to the following detailed description taken together with the accompanying figures.